Remark 3.7 (Equivalence of CIP-HDG for biharmonic equation with
divergence-free HDG for Stokes flow) 
It is well-known that the CIP-DG scheme [1]} for biharmonic equation
is equivalent to a divergence-free DG scheme
for Stokes flow, see [2]}.
The same conclusion can also be made for the current
CIP-HDG scheme (REF ) and the
divergence-free HDG scheme for a generalized Stokes flow [3]}.
Hence, we also obtained a robust preconditioner for
the divergence-free HDG scheme for the generalized Stokes flow with a slip
boundary condition.

